For those rider with BIG heads read on please.
I've been wearing XXXL helmets for years. Even though my head ONLY measures 62 CM/24 1/2" (which is solidly in the XXL range for most helmets) I'm forced to wear XXXL in almost every helmet I've ever tried. The exception was the Arai XD3 I used to own which was an XXL. I currently wear a Shoei RF-1100 in XXXL and it fits me well. I've worn Arai long-oval helmets for years (Signet....Profile) in XXXL.
While at the show in Long Beach last month I tried on an XXXL C3. BEST FITTING helmet I've ever had on!